1. Field of the Invention
The present invention relates to a method and a circuit for supplying local oscillation signals that are used in the demodulation and output by a digital signal processor of received signals of a plurality of receivers that are each connected to a respective antenna, and more particularly to a method and circuit for supplying local oscillation signals for accurately reproducing the differences in propagation delay of signals that have been received at antennas.
2. Description of the Related Art
Local oscillation signals are used in receivers for converting frequency. In a method and circuit of the prior art for supplying local oscillation signals, an oscillator that generates a local oscillation signal is provided for each of a plurality of receivers that are each provided with a respective antenna, and local oscillation signals are thus independently supplied to each receiver.
In a configuration in which local oscillation signals are independently supplied to receivers in this way, however, the phases of these local oscillation signals are not fixed, and such a configuration therefore cannot be applied in cases such as an adaptive array antenna system that requires the accurate detection of the phase component in the received signals of antennas.
The prior-art example shown in FIG. 1 is the configuration shown in Japanese Patent Laid-open No. 224138/1998. Receivers 102-1 to 102-n each apply, as input to mixers 103-1 to 103-n that perform frequency conversion: received signals that have been received from antennas 101-1 to 101-n that are provided for each of receivers 102-1 to 102-n; and a local oscillation signal from local oscillator 104 that is provided in common to receivers 102-1 to 102-n. The output of each of mixers 103-1 to 103-n is sent by way of A/D (analog/digital) converters 105-1 to 105-n to DSP (digital signal processor) 106.
When a local oscillation signal is received from a local oscillator as in the example shown in FIG. 1, however, the occurrence of phase error that arises from the time delays caused by differences in wiring length cannot be avoided.
As a countermeasure against the occurrence of phase error and the consequent disqualification for application in cases in which the accurate detection of the phase component is required, there exists a common synthesizer method in which oscillators are provided for each channel and local oscillation signals are distributed to each receiver, but the resulting large scale of the device is problematic.
Of the methods and circuits for supplying local oscillation signals of the above-described prior art, whether local oscillation signals are independently generated for the mixers of each respective receiver or a local oscillator is shared between a plurality of receivers as shown in FIG. 1, the lack of phase control for the input signals that are applied to the mixers results in the inability to avoid phase error of the local oscillation signals that are used in frequency conversion, and the differences in propagation delay of the received signals that are received from each of a plurality of receivers therefore cannot be accurately reproduced. The problem therefore exists that accurate control based on phase cannot be achieved. This inability prevents application to systems that require accurate phase control, such as an adaptive array antenna system.
The shared synthesizer method for solving this type of problem entails a complex configuration in which an oscillator is provided for each channel and local oscillation signals are distributed to each receiver, and therefore has the problem of entailing a large-scale device.